updated 9/24/2012 8:48:41 PM ET 2012-09-25T00:48:41

A necropsy revealed only two abnormalities in the baby panda cub that died yesterday at the National Zoo: She had fluid in her abdomen and a slightly abnormal liver. Tests are ongoing to determine an exact cause of death, with results expected in about two weeks.

"It is possible that there was an issue during embryo development and that was not an issue due to a post natal occurrence," Comizzoli said, explaining that such problems can happen in the wild too.

Only 1,600 giant pandas remain in the wild, according to estimates. Both Comizzoli and Rebecca Snyder, curator of mammals at Zoo Atlanta, said that the low population is primarily due to human-caused habitat loss and not to issues related to cub fragility.

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WATCH VIDEO: Watch as Mei Xiang and Tian Tian travel from China to the National Zoo.

Although scientists have attempted to monitor wild giant panda cub births by placing video cameras in dens, so far it is not possible to compare panda zoo birth rates with those in the wild.

Overall, captive giant panda breeding programs have been very successful, Snyder told Discovery News.

"In 1996, the captive giant panda population was 130," she said. "Now it is at 300 worldwide."

Zoo Atlanta has two adults and female Lun Lun has given birth to three cubs, two of which are still at the zoo. The oldest, a female named Mei Lan, was sent to China, where she now lives.

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"The team at the National Zoo are extremely knowledgeable and did everything that they could," Snyder said. "It's just that giant panda cubs are extremely vulnerable. They are the most altricial (born in an undeveloped state) of all placental mammals."

Both she and Comizzoli also said that there are challenges associated with getting a single male and female to mate.

Copper Aitken-Palmer, head veterinarian at the Smithsonian Conservation Biology Institute, told Discovery News that males are reproductively viable for six or more months out of the year, while females are only in the mood for about one to three days. In the wild, males often have to travel long distances to find mates. Even when a female is nearby, the couple may never mate.

"Not every pair is compatible," Snyder explained.

"We give our male Tian Tian chances to mate with Mei Xiang, but so far he has never been able to do what he has to do," Comizzoli said.

Instead, his zoo and others have then artificially inseminated their females. He was part of the medical team.

Beyond that issue, he said the sperm might never reach the egg. Even if it does, there could be embryo abnormalities. Many so-called giant panda pseudo-pregnancies may actually have resulted in a pregnancy that did not continue, with the embryo reabsorbed into the mother's body.

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Scientists have not been able to identify a chemical signal in giant panda female blood that could indicate she's pregnant. Often the only evidence is the tiny cub itself after birth.

Even after birth, the cub is at risk for infections and other problems. That's why Chinese cultural tradition holds that cubs should not be named until they are at least 100 days old.

The San Diego Zoo Global Wildlife Conservancy is eagerly awaiting that point for their 4.9-pound male cub, born July 29. He continues to do well, gaining weight and strength each week.

As for giant panda mom Mei Xiang, she is "only 14 years old and could give birth again," Comizzoli said. "In the wild, giant pandas live until 18 or 20, but due to pampered conditions, captive giant pandas may live to about 30."

© 2012 Discovery Channel


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